In the typical integrated circuit device, a silicon chip is provided with a number of regions of different characteristics to provide a number of circuit elements, and on one surface of the chip there is included an interconnection pattern of electrical conductors which interconnect the various circuit elements in a prescribed fashion to achieve a desired circuit arrangement.
Typically, the interconnection pattern comprises a plurality of conductive lines or runners, on one or more levels, which have been patterned from metallic layers deposited on a dielectric coating on the silicon chip and which make low resistance connection to the silicon chip, or between different levels, by way of openings or windows in the dielectric coating. Often in high density integrated circuits at least some of the runners are of aluminum, usually including traces of copper and silicon, and the dielectric coating comprises a silica glass, usually phosphorus-doped. In a typical design, the windows may be only several microns in diameter and with a height of between one and two microns.
To maximize the density of components in a chip, it is known to be desirable to use windows with straight vertical walls or edges. However, it is difficult to achieve consistently reliable electrical connection through windows of this kind. Accordingly, to improve the reliability of such connections, it is customary to make a so-called "nailhead" connection which comprises an enlarged square end portion which overlies the window with considerable overlapping of the window edges. For example, for a 3.5 microns diameter window, it is customary to use a 5 microns square nailhead end portion attached to a 3.5 microns runner. This extra width for the nailhead portion tends to reduce the density of elements which can conveniently be included in a chip.